Aging is not a slow, linear process, according to new research from Stanford researchers.

In fact, aging, particularly metabolism slowing and skin wrinkling, may happen in what scientists describe as “age-related waves” versus a steady climb. The study published in the Nature Aging journal found that the body’s molecules and the microbes and bacteria that make up the gut microbiome experience rapid change at two key intervals: Age 44 and 60.

“We’re not just changing gradually over time; there are some really dramatic changes,” Michael Snyder, PhD, professor of genetics at Stanford University and one of the study’s researchers, said in the university’s press coverage. “It turns out the mid-40s is a time of dramatic change, as is the early 60s. And that’s true no matter what class of molecules you look at.”

Molecular change, when the molecules and microbes increase or decrease the most, impacts the body’s risk of chronic disease, the ability to fight off disease, and the pace of aging, per the study. The changes in people’s 40s were closely tied to heart disease, skin and muscle function, and how people metabolize caffeine, fat, and alcohol. The molecular changes in peoples’ 60s play a role in oxidative stress, immune health, heart disease, caffeine metabolism, kidney disease, and skin and muscle aging. It is in line with the 12 hallmarks of aging, such as chronic inflammation and cellular senescence, that happen on a molecular level as people grow older. It also explains how older adults experience a higher sensitivity to alcohol, gain weight, and experience age-related muscle loss.

Researchers studied over 100 volunteers between the ages of 25 and 75 who donated blood or provided biological samples every few months over nearly two years, assessing changes in over 135,000 molecules and microbes. The authors concluded that a vast majority of the molecules and microbes, 81%, did not change in a linear fashion as the individual aged. The most rapid changes were in the mid-40s and early 60s for both men and women.

“This suggests that while menopause or perimenopause may contribute to the changes observed in women in their mid-40s, there are likely other, more significant factors influencing these changes in both men and women,” said Xiaotao Shen, PhD, an author of the study and assistant professor at Nanyang Technological University Singapore, in Stanford’s news release. “Identifying and studying these factors should be a priority for future research.”

While the underlying mechanisms of why rapid molecular change happens at these two key intervals is largely unknown and the study only captured data after a two-year follow up, it sends a message to pay particular attention to prioritizing your health as you hit mid-life to battle some of the molecular changes that are out of your control.

This means regular resistance training to counter muscle loss, moving daily to offset the risk of heart disease, and eating nutritious, filling whole foods to combat the metabolism dip.